Highly Polarized Alkenes as Organocatalysts for the Polymerization of Lactones and Trimethylene Carbonate

In this work, the activity of N-heterocyclic olefins (NHOs), a newly emerging class of organopolymerization catalyst, is investigated to affect the metal-free polymerization of lactones and trimethylene carbonate (TMC). A decisive structure activity relationship is revealed. While catalysts of the simplest type bearing an exocyclic =CH2 moiety polymerize L-lactide (L-LA) and delta-valerolactone (delta-VL) in a non-living and non-quantitative manner, the introduction of methyl substituents on the exocyclic carbon radically changes this behavior. 2-Isopropylidene-1,3,4,S-tetramethylimidazoline is found to be highly active for a range of monomers such as L-LA, (delta-VL, epsilon-caprolactone (epsilon-CL), and TMC, with quantitative conversion occurring within seconds with catalyst loadings of just 0.2 mol %. The high activity of this NHO further enables the ring-opening polymerization (ROP) of the macrolactone co-pentadecalactone (PDL). However, this broad applicability is offset by a lack of control over the polymerizations, including side reactions as a consequence of its strong basicity. To overcome this, a saturated, imidazolinium-derived analogue was synthesized and subsequently demonstrated to possess a harnessed reactivity which enables it to polymerize both L-LA and TMC in a controlled manner (D-M < 1.2). NMR. spectroscopic and MALDI-ToF MS experiments highlight the differences in polymerization pathways for 2-methylene-1,3,4,5-tetramethylimidazoline, in which the exocyclic carbon is not substituted, in contrast to 2-isopropylidene-1,3,4,5-tetramethylfinidazoline, with the former operating via its nucleophilicity and the latter acting as a base with enolizable delta-VL.